Phase-transfer catalysis (PTC) is known. As the chemical industry has sought to improve its processing efficiency, eliminate safety risks, and reduce its detrimental impact on the environment, PTC has become a recognized tool.
Conventional PTC methodology involves two immiscible phases—typically an aqueous polar phase and an organic nonpolar phase, a phase-transfer catalyst that is soluble in each phase, a substrate that is soluble in the nonpolar phase, and an anionic reagent that is soluble in the polar phase. The phase-transfer catalyst increases the reaction rate between the substrate and anionic reagent by shuttling back and forth between the two phases. The shuttling thereby transports anionic reagents into the organic phase—where reaction with the substrate can occur.
Quaternary ammonium and phosponium salts, with their unique capability to dissolve in both polar and nonpolar phases, are the catalysts of choice for most phase-transfer applications. Ammonium derivatives are the most commonly used, but phosphonium-based phase-transfer catalysts are also commonly used due to their relatively high thermal stability. Other phase-transfer catalysts include crown ethers and polyethylene glycols (PEG).
Some examples of well-known reactions that can be performed by PTC include: nucleophilic substitution reactions, e.g., halogenations and cyanations; alkylation and condensation reactions; oxidations and reductions; elimination reactions; and Wittig and Wittig-Homer reactions.
There are several advantages to using PTC over other reaction systems that employ only a single phase, and those advantages can include: increased reaction rates, lower reaction temperatures, and decreased production costs because costly anhydrous or aprotic solvents are not employed. Additionally, some reactions are known to occur via PTC that would not otherwise occur in a single-phase reaction system.
The overall efficiency of PTC can be influenced by a number of factors such as the steric hindrance associated with the phase-transfer catalyst, the phase-transfer catalyst's lipophilicity, and the lipophilicity of its counter ion.
Ionic liquids are known and generally understood to be made up of anions and cations. When organic cations are generated with moderately long alkyl chains and combined with haloaluminate or halophosphate counter ions, compounds result that have relatively low melting points. Organic salts that have low melting points can be used as solvents for organic reactions, and they have recently received much attention as ionic liquids in both industrial and academic settings. For example, carrying out chemical reactions in ionic liquids is of interest in the growing field of green chemistry, because ionic liquids have negligible vapor pressure. In addition, ionic liquids have also been employed in biphasic catalysis with water to immobilize a homogeneous catalyst in an organic phase. The chemistry of ionic liquids has been reviewed extensively.
There is therefore a need for additional phase-transfer catalysts and additional ionic liquids because of the commercial demand for both.